Process for inducing superplasticity in zinc or zinc-aluminum alloys containing copper

ABSTRACT

Process for imparting superplasticity to Zn or Zn-Al alloys containing 6-12 percent Cu by warm working an ingot of the metal until substantially the entire cross section has been worked and reduced to a thickness suitable for thermoforming. Stock so prepared can be thereafter formed with the usual superplastic deformation techniques, and resulting manufactures exhibit superior resistance to creep and corrosion and exhibit greater tensile strength.

United States Patent [72] Inventors James C. Marshall;

Terrence J. Stewart, both of Apalachin,

[21] Appl. No. 21,309

[22] Filed Mar. 20, 1970 [45] Patented Jan. 4, 1972 [73] Assignee International Business Machines Corporation Armonk, N.Y.

[54} PROCESS FOR I'NDUCING SUPERPLASTICITY IN ZINC OR ZINC-ALUMINUM ALLOYS CONTAINING COPPER 13 Claims, 2 Drawing Figs.

[52] U.S.C1 148/11.5 R [51] lnt.Cl C2211/16 [50] Field ofSearch 148/] 1.5

[56] References Cited UNlTED STATES PATENTS 3,340,101 9/1967 Fields, Jr. et al... 148/1 1.5 R 3,420,717 1/ 1969 Fields, Jr. et al 148/1 1.5 R

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorneys-Hanifin and Jancin and K. P. Johnson ABSTRACT: Process for imparting superplasticity to Zn or Zn-Al alloys containing 6-12 percent Cu by wann working an ingot of the metal until substantially the entire cross section has been worked and reduced to a thickness suitable for thermoforming. Stock so prepared can be thereafter formed with the usual superplastic deformation techniques, and resulting manufactures exhibit superior resistance to creep and corrosion and exhibit greater tensile strength.

PATENTED JAN 4 I972 STRESS (PSI) STRAIN RATEHN/lN/MIN) FIG.

INVENTORS JAMES C. MARSHALL TERRENCE J. STEWART TOR/VEY PROCESS FOR INDUCING SUPERPLASTICITY IN ZINC OR ZINC-ALUMINUM ALLOYS CONTAINING COPPER Particular metals and alloys thereof have been exploited for their hyper-extensibility with relatively low forming forces. These metals are generally designated as superplastic, and are desirable for inexpensively forming otherwise expensive or complex par'ts. Such metals, or course, have drawbacks that restrict their use in certain applications. For example, zincaluminum alloys have poor creep resistance and low tensile strength. Alloys from other bases suffer from high forming temperatures.

Metals generally classified as superplastic can be subjected to unusual tensile elongations of greater than l percent without failure due to necking, and while maintaining substantially uniform cross section. Elongations in excess of 1,000 percent have been experienced. Several superplastic alloys and the manner in which their deformation can be accomplished are described in US. Pat. No. 3,340, 101 to D. S. Fields, Jr. et al., and a processing technique for softening certain of these superplastic alloys is described in US. Pat. No. 3,420,717 to D. S. Fields, Jr. et al.

An indication of the susceptability of a metal to superplastic deformation techniques is apparent from the value of m which is the index of strain rate sensitivity in the expression where o-represents stress in pounds per unitarea, 5 represents strain rate in terms of change per unit gauge length per unit time, and K represents a proportionality constant which is approximately equal to the stress required to deform the material at unit strain rate. The value of K depends upon specific dimensions for the other variables. Values for m can be determined by either of two tests; torsionally as described in an article Determination of Strain-Hardening Characteristics by Torsion Testing by D. S. Fields, Jr. and W. A. Backofen, published in the Proceedings of the ASTM, 1957, Volume 57, pages 1259-1 ,272, and by tensile testing described in an article Superplasticity in an Aluminum Zinc Alloy" by W. A. Backofen, l. R. Turner and D. H. Avery, published in Transactions of the ASM, 1964, Volume 57 at pages 980-990.

The value of m in those metals usually employed for superplastic deformation range from 0.3 to 0.8 in which an increasing value indicates greater surface area increase with more uniform thickness. These values are an index of the total available deformation and thickness uniformity. Values of 0.3 and over indicate that satisfactory complex shapes may be formed.

All metals within the above range will exhibit deformation in accordance with an applied load-time relationship in which one can be decreased with a corresponding increase in the other. The magnitude of K, as mentioned above, is an index of this relationship. Increasing values of K indicate greater energy requirements for deformation, although the value of m also affects these requirements.

Examples of metal alloys found to be hyperextensible, hence well-suited to deformation by super plastic techniques, are indicated as 78 to 80 zinc with to 22 percent aluminum and 66 to 68 percent with 32 to 34 percent copper. (Percentages are by weight.) The first alloy is the one most used. The second is brittle at room temperature and has a narrow working temperature. Brass (zinc with 52 percent to 62 percent copper) has desirable values for m and K but cannot be formed by superplastic processes.

Most zinc-aluminum alloys can contain small amounts of other metals without deteriment to the superplastic characteristics. For instance, manganese or magnesium can be added in amounts up to 0.3 percent, and chromium or nickel can be added up to 0.5 percent. These metals usually improve alloy strength. Copper in small amounts, usually below 1 percent, has been added to these alloys because of its known effect in reducing and improving resistance to creep. Copper additions beyond this limit are readily noticeable by the greater forming energies necessary. Cold rolling reductions are limited to approximately 25 percent when these minor additions are present, compared to the 75-90 percent usually possible with the binary zinc-aluminum.

The process described in theaforementioned US; Pat. No. 3,420,717 discloses a process for softening the'eutectoid alloy of zinc and aluminum. This-process, however, is inapplicable to noneutectoid alloys.

Accordingly, a primary object of out invention isto provide a treatment process for metals containing significant amounts of copper to achieve the characteristics of superplasticity similar to those in the alloys having only minor quantities of copper.

Another important object of our invention is to provide a process for treating metals which will enable the fabrication thereof by the usual superplastic deformation techniques and possess markedly superior resistance to creep and corrosion and exhibit greater tensile'streng'th.

A further object of our invention is to provide a processing technique for zinc-aluminum alloys with a relatively high copper content which will maintain m and K substantially at those values determined for a zinc-aluminum alloy without copper and thus enable the use of conventional superplastic deformation practices during fabrications.

These and other objects of out invention willbe apparent wherein reference is made to the accompanying drawings in which:

FIG. 1 is a composition diagram of an exemplary alloy range suitable for treatment in accordance with the invention; and

FIG. 2 is a comparative data plot of stress vs. strain rate to demonstrate the effect of m and K on the energy requirement for superplastic deformation.

In the following description, alloys are considered as superplastic when they can be subjected to unusual elongations without necking and with a substantially uniform thickness after superplastic deformation having a variation within 10 percent.

We have discovered that zinc alloys containing, by weight 6-l2 percent copper and 0-36 percent aluminum can be made superplastic within the above definition. Heretofore such alloys have exhibited failure, excessive forming forces and poor thickness uniformity.

We have found that these alloys become superplastic when subjected to large amounts of warm reduction, preferably greater than percent. Reduction of such magnitude works the entire cross section of the metal. The alloys can be worked from the as-cast ingots without the necessity of quenching or heat treating. Warm reduction has been found effective if done at temperatures above room temperature and below either the transformation temperature or the temperature at which significant grain growth occurs during'processing. Significant grain growth causes an excessive increase in forming energies. The rate of grain growth in the zinc-copper alloys becomes high at temperature above 700 F.

There are, of course, preferred temperatures for performing the reductions, but these are not critical. The proportion of aluminum appears to be significant in selecting the working temperature. When the aluminum content is greater than approximately l5 percent warm working between approximately 300 and 400 F. is desirable. The maximum temperature is 530 F. due to transformation above that limit. As the aluminum content decreases, the preferred working range is broadened and raised to between approximately 300 and 500 F; an upper limit in this case is 600 F. due to grain growth. In alloys containing only zinc and copper the acceptable temperature can reach 700 F.

Warm reduction is not limited to particulartypes of working. The working can be accomplished by extrusion, rolling or other standard techniques such as swaging or drawing. Extrusion, however, appears to be more effective than rolling or swaging for those alloys having low amounts of aluminum, such as below about 15 percent It has been found that lower values of the strain rate coefficient, K, can be obtained. Alloys having a larger proportion of aluminum do not shown any significant change in K when the method of working is changed.

Although quenching can be eliminated from the usual process of producing superplastic stock, it has been found 1n the table above, the value of the strain rate coefficient, K, is generally higher for the zinc-copper alloys than for the standard zinc-22 percent aluminum. The value of K is approximately equal to the stress required to deform the material at a beneficial in lowering the value of K when the aluminum con- 5 strain rate of l in./in./min. Although the higher K indicates tent again is gr at t an abOut as in the 6888 Of that higher forming forces will probably be required, the zincsion. This effect on alloys with less aluminum 15 only minor. copper ll h a counter b l i advantage i h h Homogehllattoh treatments of these alloys Should be Short strain rate sensitivity, in is also higher. The effect of the two inor not used at all. All of the alloys ar mul iple-ph a all 1 0 creasing values is illustrated in FIG. 2 where curves l0 and 11 temperatur s, and eXCeSSWe Phase growth Occurs during the are a plot of stress vs. strain rate for the standard zinc-22 pertleatmehtcent aluminum and the second alloy in the table, zinc-8 perwe halletouhd that the alloys tespehdlhg to our reduetleh cent copper, respectively. The value of m is the slope of the Process be In the e area Show" i the eompesmeh curve for the alloy, and thus curve 11 is steeper for the zincgtam of e hmlts are approximately from 0 Percent 15 copper. The strain rates below the cross over point for the to 36 Percent ahlmthum and 6 Pe to 12 Pe p curves indicate the stress required to deform the zinc-copper with Zinc composing h l' m Aluminum Content alloy is less than that for the zinc-aluminum alloy. Therefore, beyond 36 Percent Increases the Value to tetattvely high when evaluating the relative ease of forming two superplustic Value 50 that forming forces become e high and lmpractteatalloys, both K and m must be considered and a choice made when pp content exceeds approxlmately P ah dependent upon the thermoforming rate desired or force embrittling compound becomes excessive. Alloys with a il bl c pper C nt nt below 6 p r Will res-Pond, although inefh- The alloys can have random impurities in small amounts or ciently to the conventional super-plastic forming m h other metals can be added in limited quantities to improve without special treatment. particular characteristics. Examples of these are magnesium, Comparison of several characteristics of various zincm n e, hromi m a d i kel whi h m y b add d i copper-aluminum alloys treated in accordance with ur warm amounts up to 1 percent. Preferred amounts are 0.01 percent reduction process is given below in the table. As a control for magnesium 0.3 percent for manganese, 0.3 percent for these characteristics are also given for the standard 78 percent chromium and 1 percent for nickel. zinc-22 percent aluminum alloy, which is commonly used for Mmorpaddltlons of magnoslum, manganese or chromium superplastic forming, and for 100 percent zinc. have been found to produce significant improvement in the TABLE Composition, percent by weight K, Temp., E1ong.. Zn Al Cu Other p.s.i. m percent Process 44 515 1, 000 W R 71 700 200 WE/W R .50 515 l,000 WEfWR .63 600 1,000 WE/WR. 700 200 WEIW R .31 600 1,0o0 WE/WR .42 700 200 WE/WR .62 700 000 WE/WR. .63 600 200 WE/WR .72 050 200 WE/WR B0 600 1, 000 WE/W R. .68 050 200 wE/wu .37 515 200 WR .30 515 200 Wlt .30 515 200 WR .37 515 200 WR .30 515 200 WR .34 .515 200 WR .41 515 200 WR .24 700 5a WE/WR WR=warm rolled and WE/W R=warm extrusion followed by warm rolling.

Temperatures specified in the table are those at which superplastic tensile elongation tests were conducted. The elongations attained during the tests are indicated as either greater than 200 or 1,000 percent because the tests were terminated at those values. A specimen successfully concluded the elongation set forth and would have a limit of elongation at some value exceeding that given. Alloys of each general group were subjected to a deformation test by forming in a rectangular die 3 in. x 3 in. X5 in. Formation was accomplished according to the technique of US. Pat. No. 3,340,101 above.

The specimens were all prepared from cast ingots of 1.5 in. diameter which were subjected to either warm extrusion of warm rolling, or both, as indicated. For those extruded, the ingot was reduced from 1.5 in. to 0.3 in. diameter by extrusion and then warm rolled to 0.05 in. thickness. For those warmrolled, the ingot was side forged to 0.4 in. and warm rolled to 0.05 in. thickness Generally extrusion has been found more effective for thorough working.

From the table it will be noted that several alloys exhibited a strain rate sensitivity m considerably in excess of -b 0.44 for the common zinc-alloy. This, of course, indicates their adaptability to the formation of complex shapes with a retention of uniform thickness.

creep resistance of zinc-copper alloys. A .zinc-copper alloy with 8-10 percent-copper without these additions exhibit a creep rate from 75 percent to percent per 10,000 hours at a loading of 10,000 p.s.i. With the additives present the creep rate is radically reduced to less than 0.1 percent under the same conditions. These rates are significantly better than the creep rate of 10 percent for the standard heat-treated zincaluminum alloy. Impurity additions like the foregoing have the same effect when added to alloys containing aluminum, copper in the same proportion, and less zinc. Without these additions, he creep rate of zinc-copper-aluminum alloys approximates that of the standard zinc-aluminum.

Tensile strength also improves with the additions of copper. While the standard zinc-aluminum eutectoid alloy has a tensile strength of approximately 43,000 p.s.i., that of alloys containing 8-12 percent copper was from 53,000 to 56,000 p.s.i. The presence of the above minor additives further increased this strength to a range of 56,000 to 78,000 p.s.i.

Alloys of zinc-copper have been found to not require heat treatment after thermoforming. This. advantageously eliminates a processing step-in fabrication. When the alloy contains aluminum, however, the benefit of heat treatment increases in proportion to the aluminum content.

The foregoing has described 'a process of treatment for alloys of zinc or zinc-aluminum containing 6-l2 percent copper that is effective to induce superplasticity heretofore unattainable. This process now enables a selection of superplastic alloys having significant increase in tensile strength and creep re sistance long required in the industry. With minor adjustment of processing temperatures these alloys can all be subjected to the usual superplastic deformation techniques such as those described in the aforementioned US. Pat. No. 3,340,101 to D. S. Fields, Jr. et al. The aluminum-containing alloys are preferably formed at a temperature between 500 F. and 650 F. while those without aluminum are formed between 550 F. and 700 F.

The invention offers an important advantage with regard to secondary processing steps. The alloys having less than approximately 5 percent aluminum can be subjected to such secondary steps as plating, painting, welding and soldering using standard zinc die cast procedures. Additionally the forming temperatures are kept low because alloys are zincbased.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A process of producing stock suitable for superplastic deformation from a zinc alloy nominally containing, by weight, from 6 to 12 percent copper and 0 percent to 36 percent aluminum comprising the steps of:

producing an ingot of said alloy; and

reducing said ingot by warm working until the entire cross section has been worked and decreased to a thickness for superplastic thermoforming.

2. A process as set forth in claim 1 wherein said warm working reduces said ingot to approximately percent of its original thickness.

3. A process as set forth in claim 1 wherein said warm working is done at a temperature between approximately 300 F. and the transformation temperature of said alloy.

4. A process as defined in claim 2 wherein said warm working includes extrusion of said ingot.

5. A process of making metal forms comprising the steps of:

providing a body of stock prepared in accordance with claim 1;

heating said body to a temperature just below its transformation temperature; and

forming said body while at temperatures just below said transformation temperature.

6. A method of making metal forms as defined in claim 5 wherein said body is in sheet form and said forming step comprising at least partially the step of applying a pneumatic load thereto to cause a substantial biaxial tensile deformation thereof.

7. A process of producing stock suitable for superplastic deformation from a zinc alloy nominally comprising from 6 to 12 percent copper, by weight, comprising the steps of:

producing an ingot of said alloy; and

reducing said ingot by warm working until the entire cross section has been worked and decreased to a thickness suitable for superplastic thermoforming.

8. The process set forth in claim 7 wherein said warm working reduces said ingot to at least 10 percent of its original thickness.

9. The process set forth in claim 7 wherein said warm working is down at a temperature between 300 F. and the melting point of said alloy.

10. The process set forth in claim 8 wherein said warm working includes extrusion of said ingot.

11. A process of making metal forms comprising the steps of:

providing a body of stock prepared in accordance with claim 7',

heating said body to a temperature between 550 F. and

700 F; and

deforming said body into the desired configuration while at said temperature by pressure on said body.

12. The process as described in claim 11 wherein said deformation includes applying a pneumatic load to said alloy to cause a substantial biaxial deformation thereof.

13. The process described in claim 5 wherein said forming temperatures are between 500 F. and 650 F. 

2. A process as set forth in claim 1 wherein said warm working reduces said ingot to approximately 10 percent of its original thickness.
 3. A process as set forth in claim 1 wherein said warm working is done at a temperature between approximately 300* F. and the transformation temperature of said alloy.
 4. A process as defined in claim 2 wherein said warm working includes extrusion of said ingot.
 5. A process of making metal forms comprising the steps of: providing a body of stock prepared in accordance with claim 1; heating said body to a temperature just below its transformation temperature; and forming said body while at temperatures just below said transformation temperature.
 6. A method of making metal forms as defined in claim 5 wherein said body is in sheet form and said forming step comprising at least partially the step of applying a pneumatic load thereto to cause a substantial biaxial tensile deformation thereof.
 7. A process of producing stock suitable for superplastic deformation from a zinc alloy nominally comprising from 6 percent to 12 percent copper, by weight, comprising the steps of: producing an ingot of said alloy; and reducing said ingot by warm working until the entire cross section has been worked and decreased to a thickness suitable for superplastic thermoforming.
 8. The process set forth in claim 7 wherein said warm working reduces said ingot to at least 10 percent of its original thickness.
 9. The process set forth in claim 7 wherein said warm working is done at a temperature between 300* F. and the melting point of said alloy.
 10. The process set forth in claim 8 wherein said warm working includes extrusion of said ingot.
 11. A process of making metal forms comprising the steps of: providing a body of stock prepared in accordance with claim 7; heating said body to a temperature between 550* F. and 700* F; and deforming said body into the desired configuration while at said temperature by pressure oN said body.
 12. The process as described in claim 11 wherein said deformation includes applying a pneumatic load to said alloy to cause a substantial biaxial deformation thereof.
 13. The process described in claim 5 wherein said forming temperatures are between 500* F. and 650* F. 